A structure is described for supporting concrete reinforcement bars in a concrete structure, such as a footer or foundation. The structure includes a base member having a lower surface and an opposing upper surface. A plurality of pairs of opposing first leg members extend upward from the upper surface of the base member. Each of the first leg members have a lower end connected to the base member and an upper end distally disposed from the lower end. The structure includes a plurality of cradles for receiving the reinforcement bars, where each cradle is attached to the upper ends of a corresponding pair of the opposing first leg members. In a preferred embodiment, the structure includes horizontal support members disposed between and connecting the cradles. To retain the reinforcement bars within the cradles, preferred embodiments of the structure include retaining members that protrude inward from the inner surfaces of the opposing sidewalls. These retaining members offer interference to any upward movement of the reinforcement bars. Preferably, the base member, opposing leg members, cradles, retaining members, and horizontal support members comprise a unitary structural element, such as a continuous piece of thermoplastic material formed by injection molding.